C# integration tests with Entity Framework and async methods

I'm using separate test database for testing buisness logic purposes. My business logic however consists of mainly asynchronous methods which await on other methods multiple times. I'm having problems with testing such methods and I'm running out of ideas what's the cause and how to fix it...

Here's an example of a buisness logic method that I want to test:

public async Task<string> RegisterNewUser(string name, string surname, string address, string city, string phone, string email, string password, string repeatedPassword)
{
    string errorMessage = null;

    Person newUser = new Person();

    // _context is my instance of my class inherited from DBContext
    //Getting the next ID for new user
    if (_context.People.Any())
        newUser.Id = await _context.People.MaxAsync(record => record.Id) + 1;
    else
        newUser.Id = 0;

    newUser.Name = name;
    newUser.Surname = surname;
    newUser.Address = address;
    newUser.City = city;
    newUser.Phone = phone;
    newUser.Email = email;
    newUser.Password = password;

    bool validationSuccessful = true;

    if (await _context.People.CountAsync(p => p.Email == newUser.Email) > 0)
    {
        errorMessage = "Given email address is already taken";
        validationSuccessful = false;
    }

    if (validationSuccessful)
    {
        try
        {
            // Adding user to database
            newUser.Password = GetHashedPassword(newUser.Password);
            _context.People.Add(newUser);

            // Adding activation info to database
            RegistrationActivation act = new RegistrationActivation() { PersonId = newUser.Id, ActivationKey = "blabla"};
            _context.RegistrationActivations.Add(act);

            await _context.SaveChangesAsync();
        }
        catch (Exception e)
        {
             Exception exc = e;

             while (exc.InnerException != null)
             {
                 exc = exc.InnerException;
                 errorMessage = "Exception - " + exc.Message;
             }
        }

        return errorMessage;
    }
}

Here's my actual test method:

[TestMethod]
public void Login()
{
    Person registered = PersonTestData.CreateGoodTestUser();

    string error = UnitOfWork.CrudServices.MyPersonRepository.RegisterNewUser
                (registered.Name, registered.Surname, registered.Address, registered.City, registered.Phone, registered.Email, registered.Password, registered.Password).Result;

    Assert.IsTrue(error == null, error);
}

UnitOfWork in the code above is just an object instantiated for each test method at the begining and disposed after it completes. It connects with the test database and gives access to buisness logic methods in repositories.

In the current form the test is going to fail with exception in RegisterNewUser with message: Awaiting operation time limit exceeded or sth like this becouse it's translated into my native language...

Now if I comment out the code for adding a user and adding activation info (4 lines right before _context.SaveChangesAsync()) the test will be passed. Furthermore if I remove all async / await functionality from the RegisterNewUser method - the test is going to be passed as well...

I would be really grateful if somebody could shed some light into this problem.